In general, four-cycle engines are classified into various types according to structures of valve systems. Personal watercraft or small vehicles are typically equipped with engines constructed in such a manner that camshafts are mounted above cylinders. Such engines are referred to as single overhead camshaft (SOHC) engines, which include a single camshaft, and double overhead camshaft (DOHC) engines, which include two camshafts.
In the SOHC type engine and the DOHC type engine, a crankshaft is coupled to camshaft(s) through a timing chain or a timing belt so that rotation of the crankshaft is transmitted to the camshaft(s). More specifically, the timing chain (or timing belt) is installed around a crank sprocket (or crank pulley) mounted on an end portion of the crankshaft, and a cam sprocket (or cam pulley) mounted on an end portion of the camshaft. Since the cam sprocket has twice as many teeth as those of the crank sprocket, the rotation of the crankshaft is transmitted to the camshaft such that the number of rotations of the camshaft becomes half as many as that of the crankshaft.
During assembly of the engine, first, the crankshaft provided with the crank sprocket is accommodated into a crankcase, and the camshaft provided with the cam sprocket is accommodated into a cylinder head. Then, the timing chain is installed around the crank sprocket and the cam sprocket, and a tensioner is caused to come into contact with the timing chain to apply a suitable tension to the timing chain. The cam pulley and the timing belt are incorporated into the engine according to a similar procedure.
The crankshaft rotates in cooperation with reciprocation of a piston coupled to the crankshaft through a connecting rod, while an intake valve and an exhaust valve operate in association with the rotation of the camshaft(s), causing an intake port and an exhaust port to open and close. In the four-cycle engine, the reciprocation of the piston is transmitted to the intake and exhaust valves through the crankshaft and the camshaft so that the piston and the intake and exhaust valves operate in association with each other.
It is necessary that the piston and the intake and exhaust valves operate in association with each other at suitable timings. More specifically, it is necessary that the intake and exhaust valves operate to open or close at timings at which the reciprocating piston is in a predetermined position. By allowing the piston and the intake and exhaust valves to suitably operate in association with each other, strokes (intake, compression, expansion, and exhaust strokes) in the interior of a combustion chamber are carried out correctly. As a result, high engine performance is obtained. Therefore, during assembly of the engine, it is necessary to incorporate a crankshaft and camshaft(s) with a correct phase difference (phase angle) between them.
However, since the timing chain is installed around the crank sprocket and the cam sprocket and then the tensioner is incorporated during assembly of the engine as described above, relative positions of the crankshaft and the camshaft may deviate from desired positions, that is, a phase difference between them may vary from a correct value, by application of the tension from the tensioner to the timing chain. As a result, engine performance may be degraded.
Japanese Laid-Open Patent Application Publication No. Hei. 11-129963 and No. Hei. 11-201011 disclose an engine equipped with a sensor configured to detect protrusions of a pulser rotor mounted on a crankshaft or a camshaft in order to detect which of the strokes the engine is traveling, and to thereby set suitable ignition timings.
In such an engine, it is possible to detect the phase difference between the crankshaft and the camshaft by using the pulser rotor and the sensor. But, the detecting precision is low because they are intended to detect which of the strokes the engine is traveling as described above. If the phase difference between the crankshaft and the camshaft is one tooth of the cam sprocket, it may be undetectable.